do you realize that we have a linear relationship between the number of seats filled and the ticket price? since each changes at a constant rate with respect to each other.

lets think of this graphically so that you get the idea. we want a function of price, so put that on the x-axis. the number of seats on the y-axis. now, plot a few points:

S is the number of seats, P is the price. S goes down by 1 when P goes up by 3

all we need to do is find the straight line that passes through these points. can you do that?